Collect relics in the following areas to unlock these secret costumes:

Camo Costume - Temple of Kharmoon
Croft Manor Costume - Midas's Palace
Golden Costume - Midas's Palace
Doppelganger Costume - Natta's Mines
Tomb Raider 1 Costume - Lost Valley
Tomb Raider Legend Costume - Mountain Caves
Tomb Raider 2 Swimsuit - Great Pyramid